Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes at least one stimulation lead implanted at the desired stimulation site and an Implantable Pulse Generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via one or more lead extensions. Thus, electrical pulses can be delivered from the neurostimulator to the electrodes carried by the stimulation lead(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The neurostimulation system may further comprise a handheld Remote Control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Significant to the present inventions described herein, a typical IPG may be manually inactivated by the patient, e.g., to cease stimulation of the IPG during an emergency, by placing a magnet over the implanted IPG. This can be accomplished using any one of several different types of magnetically induced shut-down circuits.
For example, referring to FIG. 1, one implementation of magnetically induced shut-down circuitry 2 generally comprises a magnetic field sensing device 4, such as a reed switch or a Hall sensor, a microcontroller 6, which controls and manages the operations of the IPG, and a delay circuit 8, which introduces a delay into an input of, e.g., 200-400 μs. The output of the magnetic field sensing device 4 is coupled to an interrupt pin of the microcontroller 6, and further coupled to a reset pin of the microcontroller 6 via the delay circuit 8. Thus, when the magnetic field sensing device 4 senses a magnetic field, such as that emitted by a magnet passed over the IPG, a switch within the magnetic field sensing device 4 closes, thereby outputting a signal indicating the desire of the patient or user to cease stimulation. The signal is conveyed to the interrupt pin of the microcontroller 6, which responds by instantaneously shutting down power to the stimulation circuitry (now shown) of the IPG, thereby ceasing stimulation of the patient, as well as performing housekeeping functions, such as storing data. The signal is also conveyed to the reset pin of the microcontroller 6, which responds by rebooting itself. Significantly, the delay introduced by the delay circuit 8 into the signal output by the magnetic field sensing device 4 allows the microcontroller 6 to perform the aforementioned housekeeping functions prior to rebooting.
IPGs are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which neurostimulator is implanted may be subjected to electro-magnetic forces generated by MRI scanners, which may potentially cause damage to the neurostimulator as well as discomfort to the patient.
In particular, in MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the present of a large static magnetic field. Present-day MRI scanners can have maximum gradient strengths of 100 mT/m and much faster switching times (slew rates) of 150 mT/m/ms, which is comparable to stimulation therapy frequencies. Typical MRI scanners create gradient fields in the range of 100 Hz to 30 KHz, and radio frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and 128 MHz for a 3 Tesla scanner.
Despite the fact that a conventional IPG implanted within a patient undergoing an MRI will be automatically deactivated (i.e., the magnetic field present in the MRI scanner will be sensed by the magnetic field sensing device, thereby automatically deactivating the IPG), the strength of the gradient magnetic field may be high enough to induce voltages (5-10 Volts depending on the orientation of the lead inside the body with respect to the MRI scanner) on to the stimulation lead(s), which in turn, are seen by the IPG electronics. If these induced voltages are higher than the voltage supply rails of the IPG electronics, there could exist paths within the IPG that could induce current through the electrodes on the stimulation lead(s), which in turn, could cause unwanted stimulation to the patient due to the similar frequency band, between the MRI-generated gradient field and intended stimulation energy frequencies for therapy, as well as potentially damaging the electronics within the IPG. To elaborate further, the gradient (magnetic) field may induce electrical energy within the wires of the stimulation lead(s), which may be conveyed into the circuitry of the IPG and then out to the electrodes of the stimulation leads. For example, in a conventional neurostimulation system, an induced voltage at the connector of the IPG that is higher than IPG battery voltage (˜4-5V), may induce such unwanted stimulation currents. RF energy generated by the MRI scanner may induce electrical currents of even higher voltages within the IPG.
In one novel technique described in U.S. Provisional Patent Application Ser. No. 61/612,214, entitled “Neurostimulation System for Preventing Magnetically Induced Currents in Electronic Circuitry,” which is expressly incorporated herein by reference, voltage supply rails of the IPG electronics are increased in response to an external signal from the RC or CP that places the IPG in an MRI-mode. In order to increase the voltage supply rails of the IPG electronics, it is necessary that the IPG not be deactivated in the presence of the magnetic field generated by the MRI scanner. In one proposed method, this can be accomplished by disabling the magnetic field sensing device to prevent deactivation of the IPG. However, it may be desirable to continue to monitor the magnetic field generated by the MRI, e.g., to determine when the MRI has been initiated and/or terminated. If the magnetic field sensing device is disabled during the MRI, this function cannot be accomplished.
There, thus, remains a need to prevent an IPG from being deactivated during an MRI, while monitoring the magnetic field during the MRI.